This invention generally relates to lids for beverage cups, and is specifically concerned with a disposable beverage lid having a reduced diameter edge that snap-fits over the brim of a drinking cup, wherein the retention force of the edge around the cup brim is increased by the provision of an extended diameter flange that increases the hoop strength of the reduced diameter edge.
Disposable beverage cup lids are well known in the prior art. Such cup lids are formed from a fusible plastic sheet material, and are designed to snap-fit over the top of a disposable beverage cup to keep the liquid contents of such cups from spilling during transport. In the past, such lids were thermoformed into plastic sheet material by applying radiant heat uniformly throughout a single piece of sheet material to soften the sheet material at all points, and then pneumatically pressing the softened sheet of material against a plurality of lid molds by the creation of a partial vacuum between the sheet material and the molds. After the molding operating was completed, the vacuum pressure between the sheet material and the molds was normalized so that the sheet could be easily pulled from the molds, and the resulting lid blanks were then individually cut out of the sheet by means of a trim press having a mechanical action similar to that of a common hole punching machine for paper.
An example of a snap-on lid manufactured by this process is disclosed in U.S. Pat. No. 4,877,151 assigned to James River Corporation. Such lids generally comprise a circular closure panel circumscribed by a side wall that terminates in a reduced diameter edge. The reduced diameter edge is dimensioned so that it is capable of snap-fitting over the annular brim that circumscribes the open end of a disposable drinking cup. To provide the reduced diameter edge with sufficient resiliency to achieve its purpose, a plurality Of flutes are provided in the side wall that circumscribes the central, circular panel of the lid. A frustro-conically shaped skirt is integrally connected to the reduced diameter edge. This skirt flares outwardly and downwardly from the edge, and not only increases the hoop strength of the reduced diameter edge, but also advantageously receives, guides, and wedgingly snaps the reduced edge of the lid over the annular brim of the drinking cup when it is pressed downwardly over the open end of the cup. The free edge of the frustro-conical skirt is typically circumscribed by a very short flange having a radial length of only about 0.030 inches. Ideally, it was thought that the flange that circumscribes the outer edge of the frustro-conical skirt should be eliminated altogether, as it served no significant purpose in the lid. However, in view of the tolerances associated with the operation of the trim press, it was necessary to leave some small amount of flange to insure that no parts of the skirt were inadvertently cut during the trimming operation.
While the snap-on lid disclosed and claimed in U.S. Pat. No. 4,877,151 represented a substantial advance in the art, the inventors observed that when lids of this design were manufactured in certain types of thermoforming machinery, the retention force between the lid and the cup fell to lower than optimal levels (i.e., in some cases, under 1 pound of force). In particular, the applicants observed that when such cups were made by way of plastic web machine, which applied localized, contact-type heating to only those portions of the sheet material which were going to be deformed by the mold, the resultant stretching in the side wall area weakened the hoop strength of the restricted diameter edge that snap-fits over the brim of the beverage cup, which in turn resulted in a lower than optimal retention force between the lid and the cup.
While the applicants recognized that it would be possible to increase the hoop strength of the restricted diameter edge by either decreasing the diameter of the edge, or modifying the pattern of flutes in the side wall which determines the resiliency of the restricted edge, or even lengthening the frustro-conical skirt that is integrally connected to the edge, any of these approaches would necessitate an expensive re-machining operation on each of the lid molds. Additionally, if the restricted diameter were made smaller, or the skirt were made longer, it might be difficult, if not impossible, to remove the lid blanks from the mold. Still another solution that the applicants contemplated was to make the sheet material which forms the lids thicker. While in actual tests this approach succeeded in increasing the hoop strength of the restricted diameter without the need for making expensive changes in the lid molds, it necessitated the use of larger amounts of plastic material to produce the lid, which in turn resulted in a substantial increase in manufacturing cost.
Clearly, what is needed is a way to increase the hoop strength of the restricted diameter to at least a 1.5 pounds without the need for an expensive re-machining of the molds, and without the use of thicker sheet material.